The son of Osbern (or Obbern) de Crépon, seneschal of Normandy, FitzOsbern himself became seneschal of Normandy and in 1060 was given the lordship and castle of Bréteuil. He took a leading part both in the preparations for the Norman invasion of England and in the Battle of Hastings (1066) and was rewarded with a grant of the Isle of Wight and the earldom of Hereford, both vitally important for the defense of England.

After the Conquest, FitzOsbern held a position of the highest responsibility. In 1067 he commanded the army in King William I’s absence, was put in charge of a new castle at Norwich, and was made the king’s special representative in the north. In the critical rebellion of 1068–69 he was governor of York. In 1071, having been sent to Normandy to help Queen Matilda, he became involved in the Flemish succession dispute and was killed at the Battle of Cassel in Flanders in 1071.

He founded the abbeys of Cormeilles and Lire in Normandy and introduced the “laws of Bréteuil” to Hereford, whence they became a model for many western English, Welsh, and Irish boroughs.

On his death, his estates were divided between his two sons—William (or Guillaume), the elder, succeeding to the Norman fiefs, and Roger Fitzwilliam, the younger, succeeding to the earldom of Hereford and the English estates. The latter conspired against King William I and in 1075 forfeited his estates and was imprisoned for the remainder of his life.